20 ' BYZANTINE CHURCHES CHAP.
shafts are substituted for the piers, a little adjustment will produce the beautiful form found in the side-chapels of the Pammakaristos (p. 152), and of S. Saviour in the Chora (p. 310)5 where the two side lights are covered by half-arches whose crowns abut on the capitals of the shafts^ while between and above them rises the semicircular head of the central light.
The method of grouping three arched windows of the same height is adopted in apse windows, each of them occupying one side of the exterior. As the deep, narrow mullions are set radiating, the arch is narrower inside than outside. But this difficulty was overcome, partly by lowering the inner crowns, so that the arch is conical, partly by winding the surface. In the Pantokrator (p. 238)3 instead of radiating to the centre of the apse, the side and mullions are placed parallel to the axis of the church, thus obviating all difficulty. Generally the centre to which the mullions radiate is considerably beyond the apse, so that any necessary little adjustment of the arch could easily be made.
Triple windows supported on circular columns are not infrequent in the north and south cross arms. Sometimes the central light is larger than the lateral lights, at other times, as in the Pantepoptes, the three lights are equal, The lower part of these windows was probably filled in with a breastwork of carved slabs, as in S. Sophia, while the upper part was filled by a pierced grille. At present the existing examples of these windows have been built up to the abaci of the capitals, but in the church of S. Mary Diaconissa (p. 18 6) the columns still show the original form on the inside.
Vaulting.—All Byzantine churches of any importance are vaulted in brick. The only exception to this rule in Constantinople is the little church known as Monastir Mesjedi (p. 264). The different systems of Byzantine vaulting have been so fully treated by Choisy and other authorities, that in the absence of any large amount of new material it is not necessary to give here more than a few notes on the application of these systems in Constantinople. It should always be kept in view that, as these vaults were